Hydrogen cyanide (HCN), also known as hydrocyanic acid, is an industrial chemical with many uses in the chemical, mining, and pharmaceutical industries. For example, HCN is a raw material for the manufacture of adiponitrile for use in producing polyamides such as nylon 66 and nylon 6. Other uses include manufacture of acrylic plastics; sodium cyanide for use in gold recovery; and intermediates in the manufacture of pesticides, agricultural products, chelating agents, and animal feed.
It is known to make HCN from compounds containing hydrogen, nitrogen, and carbon at high temperatures, with or without a catalyst. Known HCN processes include the Andrussow process, the BMA process or, to a lesser extent, the Shawinigan process. A description is provided, for example, in the Encyclopedia of Chemical Technology (Fourth Edition, Volume 7, pages 753 to 782) edited by Kirk-Othmer. Other processes for making HCN, not significantly exploited commercially due to unsatisfactory economics, include formamide decomposition, methanol ammonolysis, and reaction of acid with sodium cyanide. HCN is also a by-product of the Sohio process for the synthesis of acrylonitrile from propene and ammonia.
The Andrussow et al. endothermic HCN synthesis reaction is well-known. Thermal energy is supplied to the synthesis via exothermic co-combustion of excess methane with oxygen. U.S. Pat. No. 1,957,749 discloses the catalytic production of hydrocyanic acid (IG FARBENINDUSTRIE AG) by interaction of ammonia, a vaporizable hydrocarbon and a gas comprising free oxygen, at a temperature between 750° C. and 1250° C. and in the presence of a metallic catalyst described generally by Equation 1.CH4+NH3+1.502→HCN+3H2O  (1)The reaction proceeds at temperatures of about 1,100° C. over precious metal catalysts, typically platinum (Pt), platinum-rhodium (Pt/Rh) or platinum-iridium alloy, in gauze form at about atmospheric pressure. Other process schemes include purification of the feed streams, preheating of the feed mixtures, and use of O2 enriched air as well as 100% O2 as the oxygen source.
The BMA process for HCN synthesis is also known. U.S. Pat. No. 2,768,876 discloses the catalytic process for the production of HCN from volatile hydrocarbons and where mixtures of volatile hydrocarbons and ammonia are in the gaseous phase in the presence of a platinum metal catalyst at atmospheric pressure. The step of reacting this gas mixture is in the presence of a platinum metal catalyst in a heated reaction chamber with walls of a ceramic material composed of 0.7 to 8% of SiO2 and the remainder alumina.
The BMA process involves the endothermic reaction of methane and ammonia according to Equation 2.CH4+NH3→HCN+3H2  (2)The BMA process limits undesirable side reactions associated with other processes for making HCN by the elimination of O2 in the feed gases. In this process, methane and ammonia are reacted in the absence of oxygen at temperatures above about 1200° C. The reaction is performed in externally heated sintered alumina (ceramic) tubes within a furnace, the tubes being coated with platinum.
Koch et al., in U.S. Pat. Nos. 5,470,541 and 5,529,669, disclose a process and apparatus for making HCN, including the preparation of HCN from the reaction of ammonia vapor and a hydrocarbon gas over a platinum group metal catalyst while providing heat using single mode microwave irradiation. Substantial heat energy input is required to provide the high process temperature due to the endothermic character of the process. These disclosures are incorporated by reference herein.
U.S. Pat. Nos. 5,958,273; 6,287,531; 6,315,972; and 7,070,743 disclose, collectively, a process and apparatus for making HCN, including the preparation of HCN from the reaction of ammonia vapor and a hydrocarbon gas over a platinum group metal catalyst while using induction heating as a source of energy. These disclosures are incorporated by reference herein.
The elevated temperatures employed for these gas-phase reactions (when catalyzed using one or more platinum group metals suffer from shortened catalyst life. This observation in the Andrussow process may be associated with rapid restructuring of the catalyst structure and a higher pressure drop.
In the foregoing processes, the emerging product stream must be promptly cooled below about 300° C. to prevent thermal degradation of the HCN. Additionally, unreacted ammonia, “ammonia breakthrough,” must be removed. Ammonia may catalyze the polymerization of HCN, a process hazard. Typically, the ammonia is recovered and recycled; in smaller units it may be flared or removed as ammonium sulfate.
Heat pipes, also known as heat tubes, are sealed hollow tubes of a thermoconductive metal, containing a small quantity of a working fluid to transport thermal energy primarily through vaporization and condensation. The hollow tube that contains the working fluid is also known as the heat tube envelope.
The principle of a heat pipe employing the benefits of capillary action were noted by George Grover et al. at Los Alamos National Laboratory and subsequently published by Grover, G. M., T. P. Cotter, and G. F. Erickson in the Journal of Applied Physics 35 (6) 1964; “Structures of Very High Thermal Conductance.”
Internally, the heat pipe's tubing may have wick structure on the side-walls exerting a capillary force on the liquid phase of the working fluid. This wick structure is typically a sintered metal powder or a series of grooves parallel to the tube axis, but it may in principle be any material capable of soaking up the working fluid. If the heat pipe has a continual slope with the heated end down, a wick structure is optional. Heat pipes have no moving parts.
The heat pipe mechanism relies on a vapor pressure over the hot liquid working fluid at the hot end of the pipe to be higher than the equilibrium vapor pressure over condensing working fluid at the cooler end of the pipe. It is this pressure difference that drives a rapid mass transfer to the condensing end where the excess vapor condenses and releases its latent heat. The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. Because the heat pipe is evacuated and then charged with the working fluid prior to being sealed, the internal pressure is set by the vapor pressure of the working fluid. Applications of heat pipes well-known in the art include: cooling systems, in space craft as a means for thermal management of internal temperature and in computer systems to remove heat from high speed central processor units.
A heat pipe can be used for conveying thermal energy to or away from a chemical reactor. Examples of such heat pipe applications are disclosed in at least U.S. Pat. No. 4,351,806 and U.S. Pat. No. 4,315,893.
U.S. Pat. No. 7,069,978 to Rosenfeld, et al. discloses problems with efforts to use aluminum envelope material and water as the preferred working fluid because aluminum oxide is compatible with water, even though aluminum metal is not compatible. The large difference in thermal expansion between aluminum and its oxide and resulting stresses cause the oxide layer to crack, often on the first thermal cycle, resulting in heat pipe failure. The '978 patent proposes a magnesium alloy envelope with a protective layer formed on the inside wall of the envelope to be compatible with the working fluid.
While HCN can be produced by the reaction of methane and ammonia in the presence of a platinum group metal catalyst, there remains a need to improve the thermal efficiency of the basic reaction and thus the economics of HCN production. Particularly important is the minimization of thermal energy use and ammonia breakthrough in maximizing the HCN production rate. Additional considerations include the preservation of the catalyst integrity and amount of precious metal catalyst in the reaction zone.
However, as discussed above, the commercial production of HCN occurs at elevated temperatures of about 1,100° C. over precious metal catalysts.
Heat pipe metallurgy and working fluids, axial heat flux and surface heat flux, are disclosed in “How to Select a Heat Pipe”, Enertron, Inc. (available from http://www.enertron-inc.com/enertron-resources/library.php). In particular, sodium working fluid and nickel, stainless steel vessel or envelope material are disclosed as suitable for temperature ranges of 500° C. to 900° C. Lithium working fluid and niobium+1% zirconium vessel or envelope material are disclosed as suitable for temperature ranges of 900° C. to 1500° C.
It would be uneconomical to design a commercial reaction vessel of exotic and expensive materials, despite other possible benefits flowing from the use of a heat pipe in a high temperature process.
An economically feasible, commercial reaction vessel, using affordable materials of construction to make a heat pipe device suitable for use at high temperatures, would be highly desirable. Methods of conducting various chemical reaction processes employing such a heat pipe device and reaction vessel for high temperature chemical reactions would also be highly desirable.